1. Field of the Invention
The present invention relates to a synchronous detection method, and a device, that are used to sample a signal component synchronous with a reference signal from an input signal, and to a sensor signal detector utilizing the synchronous detection.
2. Description of the Related Art
In the past, a synchronous detection circuit has been used to separate a desired signal component from an input signal that suffers a poor signal-to-noise ratio or to detect a phase difference of an input signal from a reference signal.
The synchronous detection circuit performs full-wave rectification on a signal, of which the frequency falls within a predetermined frequency band, using a reference signal of the frequency, and thus restores a desired signal. The synchronous detection circuit has a drawback that switching of connections through a switch and selection of a signal path are synchronized with the reference signal and this causes high-frequency noise.
The high-frequency noise degrades the signal-to-noise ratio the circuit offers. Normally, a low-pass filter such as a CR filter is installed in a stage succeeding the synchronous detection circuit. The low-pass filter removes the high-frequency noise.
However, when an attempt is made, as conventionally, to minimize the high-frequency noise, which occurs in the synchronous detection circuit, using the low-pass filter including the CR filter, a time constant required by the low-pass filter must be increased. For this purpose, the capacitance of a capacitor C included in the low-pass filter and the resistance of a resistor R included therein must be increased of the number of stages each having a capacitor and resistor connected to each other (or in other words, the order of the filter must be raised).
Consequently, the conventional synchronous detection circuit has a drawback that it cannot be designed to be compact and low-cost because of the low-pass filter included for removing the high-frequency noise.